1. Technical Field
The present invention generally relates to regulating the flow of material within rotary equipment, and more particularly, to a method and apparatus for controlling the amount of material picked up and veiled while drying aggregate materials, such as asphalt, within a dryer drum.
2. Background of the Invention
Asphalt is typically produced by heat drying virgin asphalt aggregate and by adding to it and mixing with it liquid asphalt cement, fillers and other additives, often including recycled asphalt pavement. Often times, asphalt is also made by drying virgin asphalt aggregate and moving it to a batch plant tower for batch mixing with the asphalt and other additives.
Asphalt manufacturing machines producing paving compositions are well-known. Generally, such machines include an area or chamber for heating and drying the aggregate and a area or chamber for mixing the heated and dried aggregate together with other materials, such as asphalt cement, liquid asphalt, fines, etc. There are generally two types of mechanisms for making aggregate dryers, that being parallel flow and counterflow. In a parallel flow dryer, cold, wet material is introduced into the cylindrical dryer near the combustion zone. The material to be dried is continuously fed into the upstream end of the dryer where the burner is located. By contrast, the burner of a counterflow dryer is located at the downstream or discharge end of the dryer. The cold, wet material enters the dryer on the upstream end and moisture is gradually driven off and the aggregate temperature is raised as the material progresses downstream in the dryer. A drying drum, dryer or cylinder is where particles are moved generally from the front to the back while being heated by a centrally located burner to dry. Within the drum, fixed flights are attached to the interior cylinder wall to stir the mixture. These flights are the internal xe2x80x9cpaddlesxe2x80x9d secured to the dryer""s wall used to deflect and direct the mixture/aggregate as the aggregate moves and travels through the dryer. Flights also enhance the mixing of the mixture and prevent the mixture from sticking to the interior wall or around the wall. These flights pick up the mixture at the bottom of the rotating drum and drop the mixture as the drum rotates. The amount and specific locations of the dropped mixture will depend on the material being dried, the speed of the rotation and the profiles of the flights. Often times the pattern of the flowing and cascading mixture within the drum caused by the flights is referred to as the veil or the veil profile because the mixture takes on a certain profile throughout the drum. Generally, the veiling aggregate is more dense on the uphill side of the rotating drum than the downhill side of the drum.
As the aggregate is tumbled and dried in the dryer, dust is naturally created and carried by the hot gases of combustion. Emission regulations prohibit the discharge of such gases with dust to the atmosphere. In addition, depending on the speed of rotation and the temperature of the dryer, the dust created may represent a portion of the fine aggregate material needed in the particular mix. As a result, dust collection or recovery systems, such as baghouses and cyclone separators, are used for the removal of the dust before further processing of the gases and exhaustion to the atmosphere. The dust and gas conveyed to a baghouse or other similar air or gas filtration means are separated; the dust is separated collected for later use while the gases are vented to the atmosphere.
The interior of such a dryer can reach 300xc2x0 F.-500xc2x0 F. Typically, in a counterflow system, the air temperature in the drum is around 400xc2x0 F. with the temperature in this region of the burner reaching up to 3,000 degrees Fahrenheit. To reach the maximum rated capacity of an asphalt plant, the exhaust temperature exiting the stack of the baghouse should be in the range of between 220xc2x0 F. and 250xc2x0 F. at about 70xc2x0 F. ambient temperature. Over time, it has been observed that the stream of gasses will loose about 10xc2x0 F. to 30xc2x0 F. from the time it exits the dryer and passes through the duct work and baghouse. By controlling the temperature in the baghouse stack, one can control the efficiency of the dryer. In short, it has been found that one way to control efficiency of the aggregate dryer, or mixer drum, is to control the stack temperature of the baghouse.
Compounding the above observations, it has been noted that if the exhaust stream entering the baghouse exceeds 250xc2x0 F., energy is being wasted. If the temperature exceeds 350xc2x0 F., the nomax bags used for filtering the air can be damaged and loose their filtering ability. Replacing the bags is expensive.
Further, aggravating the above, unfortunately, mixer drums or aggregate dryers are not run at the same levels all of the time. They are operated in a wide range of production rates. If, for example, an operator reduces a production level to fifty percent (50%), the exhaust temperature generally will fall below 200xc2x0 F. This creates a dangerous condition because if the exhaust temperature drops below 180xc2x0 F., moisture can accumulate in the baghouse. Moisture combined with dust form mud which can blind the bag, effectively shutting down the baghouse.
One way to control the baghouse stack""s temperature is to adjust the aggregate being picked up and veiled; in short, controlling the veil. This is accomplished by manipulating the flights to change the drum""s profile. Thus, if one can control the flights as one adjusts the production level, one is theoretically capable of controlling the baghouse stack""s temperature and the system""s efficiency.
Over the years there have been many attempts to design variable flights. One of the most common approaches today is to have one or more people enter the dryer and physically remove some of the flights from the interior of the dryer when the production level is to be decreased. If the production level is to be increased, the flights removed are physically reattached and put back into place. This approach has many down sides. First, it is labor intensive and time consuming. Second, it is very imprecise.
In another approach, the internal flights are set or established at 75% rated capacity of the system. This approximation permits one to theoretically operate within a 50%-100% production level. However, this is not true. As the system approaches 100% capacity, the exhaust temperature will rise substantially and the efficiency of the dryer will be reduce accordingly. In the same vein, attempts have been made to remove some of the flights within the dryer to accommodate lower production levels. However, again, as production rates increase, the exhaust temperature rises substantially, diminishing system efficiency. At times, the exhaust temperatures to the baghouse become so great, that the baghouse controls shut off the burner fuel valve and hence the burner in the dryer. This is because baghouses are equipped with sensors to monitor the temperature of the exhaust gas stream and to shut off fuel to the burner when the temperature exceeds about 350xc2x0 F. A way around this is to physically add flights as production increases, a laborious task.
Other teachings learned over time include the fact that combustion flights should not carry aggregate a substantial distance up the interior side wall of a drum as the drum rotates and then allow the aggregate to fall vertically down the interior face of the combustion flighting. This is because the deflection of dust and aggregate particles can enter the combustion zone, impinging on the combustion process. The flights must also be able to withstand intense heat (2,400xc2x0 F. to 3,200xc2x0 F.) for extended periods of time without warping, distorting, or simply burning up; thus preventing the aggregate from falling between the flights and contaminating the combustion zone.
Others have tried methods and means for modifying flights. Such attempts are shown in U.S. Pat. Nos. 5,083,382 to Brashears; 5,515,620 to Butler; 3,641,683 to Preeman; 6,132,560 to Gerstenkorn; 6,110,430 to Swisher et al.; 5,558,432 to Swisher; 4,940,224 to Musil; 4,307,520 to Lutz; 4,136,966 to Mendenhall; 3,780,447 to Fales; 3,717,937 to Thompson; 3,098,797 to Graff et al.; and 1,009,225 to Cummer. The patent to Brashears discloses a rotary drum dryer including a plurality of circumferentially spaced flights. A radially inwardly directed dam is interposed between each flight section and serves as a pivotal mount for the flights. The angular position of the flights is changeable before operating the drum by locating bolts on the flights in selected detented positions in arcuate tracks. The patent to Butler discloses a method and apparatus for operating a rotatable drying drum. The drum includes identical drying flights, each having a generally U-shaped body with two differently shaped edges. By reorienting the flights within the drying zone by securing them in one of a plurality of orientations before operating the drum, the veiling patterns of the aggregate across the drum may be altered. And, the patent to Preeman discloses an asphalt plant dryer having placed circumferentially to the inner wall pivotally mounted lifter plates adjacent its inlet end. The invention includes adjustable straight lifter plates to minimize the obstruction to the flow of the hot gaseous fluid.
Each of these patents relates to variable flights, requiring an operator to make the correct adjustments to a wide variety of operating conditions. Similarly, the concept of removing flights or totally reorienting them appears quite time consuming and impractical.
As a result, there is a need for a practical method and mechanism for regulating and controlling the pick-up and veiling of materials within a dryer that is neither all consuming nor guesstimating.
The present invention relates to an apparatus and method for regulating the quantity of aggregates picked up and veiled by the flights in an aggregate dryer. It allows a plant operator to make modifications and orientations to the doors acting as flights in a minimum amount of time. By doing this, the temperature in the air stream exiting the baghouse stack is controlled and the efficiency of the system is maintained and preserved. This, in turn, saves on the fuel consumed by the system.
The development is a flight incorporating hinged doors or plates that are easily opened or closed (approximately 20 minutes). The internal configuration of the dryer can be easily changed. Each flight has a support, door, hinge and fasteners. The door is attached to the hinge and is attached to its home support or adjacent support. In particular, the door is normally attached to its xe2x80x9chomexe2x80x9d supportxe2x80x94xe2x80x9cthe open position.xe2x80x9d Optionally, the door can be attached to the xe2x80x9cadjacentxe2x80x9d supportxe2x80x94xe2x80x9cthe closed position.xe2x80x9d The positions can be changed by merely removing/loosening the connection between the door and one support adjacent the door on one side and rotating the door to the other support adjacent the door on the other side.
This opening and closing of select doors changes the drums"" profile and controls the quantity of aggregates carried up the sidewall of the drum and veiled over the cross section of the dryer shell resulting in a change in the flow patterns of the materials in the drum.
In particular, a system for controlling the veil of mixable substance is disclosed for a rotating mixing chamber with an interior surface. A plurality of radially spaced apart plates, each having a first end pivotally connected to the interior surface and a second, distal end. Each plate is optionally rotatable about the first end between an open position and a closed position. A support is disposed between each plate for selectively mating with a single plate on either one side of the support or the other side of the support. The support is connected at a first end to the interior surface and does not move. Thus, the plate is connected to one adjacent support when the plate is in the open position and the plate is connected to the other adjacent support when the plate is in the closed position. Means, such as bolts, are used to connecting the plate to either the one adjacent support or the other adjacent support.
Each plate has at least two sections, a first section adjacent the first end and the pivotal connection and a second section at a distal end thereof. Each section is substantially planar and the two sections are angularly related to one another via an obtuse angle (about 167xc2x0). When a plate is in the open position, it is connected to the closest adjacent support with the connection being made with the first section of the plate. When the plate is in the closed position, it is connected to furthest adjacent support (on the other side of the plate from the closest adjacent support) with the connection being made with the second section of the plate. The plate has a plurality of spaced apart apertures therein and the supports have a plurality of openings therein and when the plate is connected to a support, an aperture in the plate is aligned with an opening in the support, a fastener cooperating with both the aperture and the opening.
A method is further disclosed for modifying the pick-up and veiling of a mixture in a rotating drum having an interior surface. This process includes attaching a plurality of flight assemblies to the interior of the drum with each flight assembly including a plate rotatably attached to the interior surface of the drum and the plate having a first end pivotally connected to the interior surface and a movable second, distal end. The plate is rotatable about the first end between an open position and a closed position. A first support, spaced apart from one side of the plate and fixedly connected at a first end to the interior surface of the drum, is made to contact and connect with the plate when the plate is in the open position. A second support, spaced apart from the opposite side of the plate and fixedly connected at a first end to the interior surface, is made to contact and connect with the plate when the plate is in the closed position. An operator needs only to selectively fix each flight assembly to an open position or a closed position by connecting each plate to either the first support on the one side of the plate or the second support on the other side of the plate. Selectively fixing each flight assembly to an open position or a closed position is accomplished by connecting a plate in the open position by connecting the plate to the first support with the connection being made with the first section of the plate or by connecting a plate in the closed position by connecting the plate to the second support with the connection being made with the second section of the plate.
These and other aspects of the present invention set forth in the appended claims may be realized in accordance with the following disclosure with particular reference to the accompanying drawings.